The present invention relates generally to semiconductor devices and, in particular, to ultra-thin gate oxide memory devices.
A conventional metal-oxide-semiconductor (MOS) device is illustrated in FIG. 1. The device generally includes a gate electrode 20 which acts as a conductor to which an input signal is typically applied via a gate terminal (not shown). Conventionally doped active areas 14 and 16 are formed within the semiconductor substrate 10 and act as source and drain regions 14 and 16, respectively. A channel region 12 is formed in the semiconductor substrate 10 beneath the gate electrode 20 and separates the source/drain regions 14, 16. The channel region 12 is typically doped with a dopant opposite to that of the doped source/drain regions 14, 16. The gate electrode 20 is separated from the semiconductor substrate 10 by an insulating gate oxide layer 18, which is typically an oxide of silicon, for example silicon dioxide (SiO2). The gate oxide layer 18 prevents current from flowing between the gate electrode 20 and the semiconductor source region 14, the drain region 16 and/or the channel region 12.
When an input voltage is applied to the gate electrode 20, a transverse electrical field is set up in the channel region 12. By varying the transverse electrical field, the conductance of the channel region 12 between the source region 14 and the drain region 16 is modulated. This way, an electric field controls the current flow through the channel region 12. This type of device is commonly known as an MOS field-effect-transistor (MOSFET).
The growth of the gate oxide layer, such as the gate oxide layer 18 of FIG. 1, is a critical step in manufacturing miniaturized semiconductor devices. Thin gate oxide layers free of defects and of high quality without contamination are essential for proper device operation, especially when current design rules demand gate oxide layers with thicknesses of less than 15 Angstroms, and even less than 10 Angstroms. To obtain high-quality gate oxide layers, the surface of the active area of the device is typically treated with a wet etch to remove any residual oxide. The gate oxide is then grown slowly, typically through dry oxidation in a chlorine ambient. At this point, it is extremely important to carefully control the growth of the gate oxide because the thickness and uniformity of the gate oxide layer can significantly impact the overall operation of the device formed. Because the drain current in a MOS device is inversely proportional to the thickness of the gate oxide, it is desirable to make the gate oxide as thin as possible while taking into account the oxide breakdown and reliability considerations of the process. Furthermore, the use of silicon dioxide for gate oxide layers thinner than 20 Angstroms poses various problems, one of them being the leakage current caused by direct tunneling, which further affects the operation of the device.
High-dielectric constant insulating materials have been proposed as gate oxide layers, but with limited results. FIG. 2 illustrates a high-dielectric constant insulating layer 19 formed between the gate electrode 20 and the semiconductor substrate 10. Conventional high-dielectric constant insulating materials such as tantalum oxide (Ta2O5), titanium oxide (TiO2) or barium oxide (BaO), for example, are not thermally stable when in direct contact with a silicon substrate. Accordingly, these high-dielectric constant insulating materials require a diffusion barrier layer 21 (FIG. 2) at the interface with the silicon substrate, the formation of which adds process complexity.
Furthermore, using a diffusion barrier layer defeats the purposes of using a high-dielectric constant insulating material because the gate capacitance is decreased rather than increased. If the gate structure of FIG. 2 is viewed as a series of stacked capacitors 25 (FIG. 3), which has layers of thicknesses comparable to those of the gate structure of FIG. 2, then, a first capacitor C1 (FIG. 3) corresponds to the high-dielectric constant insulating layer 19 and a second capacitor C2 (FIG. 3) corresponds to the diffusion barrier layer 21. The diffusion barrier layer 21 (FIG. 2) acts as a series capacitor the addition of which decreases the capacitance of the gate electrode 20. The capacitance of the first capacitor C1 is larger than the capacitance of the second capacitor C2 and, thus, voltage V1 which occurs across the first capacitor C1 is smaller than voltage V2 which occurs across the second capacitor C2. As a result, the applied voltage V that occurs across the series capacitors 25, that is the sum of V1 and V2, appears mostly across the diffusion barrier layer 21 rather than across the high-dielectric constant insulating layer 19.
Accordingly, there is a need for an improved memory device which eliminates the problems posed by the use of a conventional high-dielectric constant insulating materials as gate oxide layers. There is also a need for an improved ultra-thin gate oxide layer which is thermally stable when in contact with silicon and which is resistive to impurity diffusion, and a novel method for its fabrication. A memory device with a minimal voltage drop across the gate electrode is also desirable, as well as a method of forming such a memory device.
The present invention provides an ultra-thin gate oxide layer of hafnium oxide (HfO2) as a thin medium-dielectric constant gate insulating layer. The ultra-thin gate oxide layer of hafnium oxide (HfO2) is formed by a two-step process: (1) a thin hafnium (Hf) film is formed by thermal evaporation at a low substrate temperature, after which (2) the thin hafnium film is radically oxidized using a krypton/oxygen (Kr/O2) high-density plasma to form the ultra-thin gate oxide layer of hafnium oxide (HfO2). The ultra-thin gate oxide layer of hafnium oxide (HfO2) formed by the method of the present invention is thermally stable in contact with silicon and is resistive to impurity diffusion at the HfO2/silicon interface. The formation of the ultra-thin gate oxide layer of hafnium oxide (HfO2) eliminates the need for a diffusion barrier layer, allows thickness uniformity of the field oxide on the isolation regions and, more importantly, preserves the atomically smooth surface of the silicon substrate.